In a conventional electronic layout, printed circuit boards are used to interconnect integrated circuits. The integrated circuits are coupled to conductive foil patterns located on the circuit boards. This interconnection level is coplanar with the circuit board and defines an X-Y interconnection plane.
When a single board cannot hold all of the circuit components, because of size or required signal flow constraints, several circuit boards may be arranged and interconnected in a number of configurations. One common high density interconnect configuration is a stack of circuit boards which are electrically interconnected with interboard connectors. This corresponds to electrical interconnection in the Z-axis. This three-dimensional packing scheme creates a compact module assembly that has a density limited primarily by heat dissipation and connector spacing requirements.
Although many interboard connectors are suitable for interconnecting stacked circuit boards. Many of these connectors limit the inter-board spacing of stacked circuit boards. This is undesirable in high speed applications where signal timing needs depend on the signal path lengths, the longest of which is termed the "critical path length." To increase circuit operating speed the critical path length must be reduced.
Z-axis connectors address this need. For example, U.S. Pat. No. 4,813,128 issued to Massopust, and assigned to the assignee of the present invention, teaches a method of interconnecting circuit boards using blocks and Z-axis connectors. This type of interconnection scheme reduces the average signal path as compared to a conventional edge type connectors. However, the Massopust structure does not minimize the critical path length because the presence of the blocks limits inter-board spacing. As a consequence, the number of connection points is comparable to the number exhibited by an edge connector.
Other Z-axis connectors are disclosed in U.S. Pat. Nos. 3,097,032 issued to Hochheiser, and 2,969,521 issued to Scoville. While the Hochheiser and Scoville inventions do not use blocks, the number of connection points to interconnect the circuit boards is similar to that of edge connectors, and the critical path length is limited by the fork height requirements. Additionally, use of the inventions requires the extra step of placing the contacts on the board.
A Z-axis connection scheme that addresses critical path length limitations is described in U.S. Pat. No. 3,212,049, issued to Mittler et al. Mittler describes a scheme where contact bushings are installed in circuit board holes and placed in electrical contact with the circuit board foil patterns. The circuit boards are then stacked and Z-axis connectors are run through the aligned bushings to interconnect the boards. While the Mittler patent allows a short critical path length, the insertion of bushings into the circuit boards adds complexity and reduces component density. Additionally, the number of contact points remains the same as in the edge connector.
Another Z-axis connection scheme that addresses the critical path length limitations is described in U.S. Pat. No. 3,867,759, issued to Siefker. Siefker describes a method of soldering Z-axis wires in place for stacked strip-line circuits.